The present invention relates to the construction of subterranean wells. More particularly, the present invention relates to methods and constructions for centering a casing within a well, particularly an oil or gas well.
A well is any boring through the Earth's surface that is designed to find and acquire liquids or gases. Wells for acquiring oil are termed “oil wells”. A well that is designed to produce mainly gas is called a “gas well”. Typically, wells are created by drilling a bore, typically 5 inches to 40 inches (12 cm to 1 meter) in diameter, into the earth with a drilling rig that rotates a drill string with an attached bit. After the hole is drilled, sections of steel pipe, commonly referred to as a “casing” and which are slightly smaller in diameter than the borehole, are dropped “downhole” into the bore for obtaining the sought after liquid or gas.
The difference in diameter of the wellbore and the casing creates an annular space. When completing oil and gas wells, it is often important to seal the annular space with cement. This cement is pumped in, often flushing out drilling mud, and allowed to harden to seal the well. To properly seal the well, the casing should be positioned so that it is in the middle or center of the annular space. The casing and cement provides structural integrity to the newly drilled wellbore in addition to isolating potentially dangerous high pressure zones from each other and from the surface. Thus, centralizing a casing inside the annular space is paramount and critical to achieve a reliable seal, and thus good zonal isolation. With the advent of deeper wells and horizontal drilling, centralizing the casing has become more important and more difficult to accomplish.
A traditional method to centralize a casing is to attach centralizers to the casing prior to its insertion into the annular space. Most traditional centralizers have wings or bows that exert force against the inside of the wellbore to keep the casing somewhat centralized. Unfortunately, these centralizers increase the profile of the casing, thereby causing increased resistance and potential snagging during casing installation.
Traditional centralizers are commonly secured at intervals along a casing string to radially offset the casing string from the wall of a borehole in which the casing string is subsequently positioned. The centralizers generally include evenly-spaced ribs that project radially outwardly from the casing string to provide the desired offset. Centralizers ideally center the casing string within the borehole to provide a generally continuous annulus between the casing string and the interior wall of the borehole. This positioning of the casing string within a borehole promotes uniform and continuous distribution of cement slurry around the casing string during the subsequent step of cementing the casing string in a portion of the borehole. Uniform cement slurry distribution results in a cement liner that reinforces the casing string, isolates the casing from corrosive formation fluids, prevents unwanted fluid flow between penetrated geologic formations, and provides axial strength.
A bow-spring centralizer is a common type of centralizer that employs flexible bow-springs as the ribs. Bow-spring centralizers typically include a pair of axially-spaced and generally aligned collars that are coupled by multiple bow-springs. The bow-springs expand outwardly from the axis of the centralizer to engage the borehole sidewall to center a pipe received axially through the generally aligned bores of the collars. Configured in this manner, the bow-springs provide stand-off from the borehole, and flex inwardly as they encounter borehole obstructions, such as tight spots or protrusions into the borehole, as the casing string is installed into the borehole. Elasticity allows the bow-springs to spring back to substantially their original shape after passing an obstruction to maintain the desired stand-off between the casing string and the borehole.
Attempts have been made to develop low-profile, deployable centralizers that can be added to the outside of the casing/pipe. These are designed to reduce friction and snagging due to the fact that the supports or bows are retracted until in their final position. The challenge in developing an effective deployable centralizer is to make it as low profile as possible, actuate deployment upon demand, and to overcome de-centralizing force.
Centralizers are usually assembled at a manufacturing facility and then shipped to the well site for installation on a casing string. The centralizers, or subassemblies thereof, may be assembled by welding or by other means such as displacing a bendable and/or deformable tab or coupon into an aperture to restrain movement of the end of a bow-spring relative to a collar. Other centralizers are assembled into their final configuration by riveting the ends of a bow-spring to a pair of spaced-apart and opposed collars. The partially or fully assembled centralizers may then be shipped in trucks or by other transportation to the well site.
U.S. Pat. No. 6,871,706 discloses a centralizer that requires a step of bending a retaining portion of the collar material into a plurality of aligned openings, each to receive one end of each bow-spring. This requires that the coupling operation be performed in a manufacturing facility using a press. The collars of the prior art centralizer are cut with a large recess adjacent to each set of aligned openings to accommodate passage of the bow-spring that is secured to the interior wall of the collar. The recess substantially decreases the mechanical integrity of the collar due to the removal of a large portion of the collar wall to accommodate the bow-springs. The collars of the casing centralizer disclosed in this patent also require several additional manufacturing steps, including the formation of both internal and external (alternating) upsets in each collar to form the aligned openings for receiving and securing bow-springs, a time-consuming process that further decreases the mechanical integrity of the collar.
U.S. Pat. No. 4,545,436 and Great Britain Patent No. 2242457 both disclose casing centralizers having a plurality of bow-springs which are connected at either end to the first and second collars. As described in U.S. Pat. No. 4,545,436, the bow-springs are connected to the collars using rivets or by welding. Conversely, in Great Britain Patent No. 2242457, the bow-springs are connected using nuts and bolts.
Improved centralizers and methods continue to be sought, particularly in view of the limitations of the prior art and the need for better and stronger centralizers. Considerations for the development of new centralizers and new methods of assembling the centralizers include manufacturing costs, shipping costs, the costs associated with installing the centralizers onto pipe strings and the ease of running the pipe string into the well.